Allogeneic H-2d leads to H-2b irradiation bone marrow chimeras: a) failure to transfer chimerism adoptively and b) immune reactivity of immunocompetent lymphocytes adoptively transferred to chimeras.

Long-lived, GVHD-free, H-2 incompatible haemopoietic chimeras (P1 leads to P2) were constructed transfusing unmanipulated bone marrow cells together with recently identified marrow-regulating factors (MRF) in lethally irradiated recipients. The chimeric tolerance of P1 leads to P2 chimeras proved to be adoptively untransferable. Another peculiar property of established P1 leads to P2 allochimeras was their ability to "suppress" or reject passively transfused immunocompetent P1 or P2 lymphocytes. Even this "suppression" appeared to be untransferable and to operate in the chimera only in a fashion dependent upon the age of the established chimeras. This chimeric "unidentified suppressive principle" seems not to follow familiar immunologic lines. A relationship with the mechanism of chimeric tolerance is suggested.